A step index optic fiber consists of a core, a cladding around the core, and a buffer coating around the cladding. The core is fabricated to obtain an index of refraction slightly greater than that of the cladding. Furthermore, the buffer coating, which may be plastic, is fabricated to obtain an index of refraction greater than that of the cladding.
Light may be transmitted along the core with little loss of light. A step index optic fiber is commonly referred to as a single-mode fiber because the original intent was to transmit light in a single way, or mode, through the core.
With the buffer coating in place, any light that escapes the core due to bending or other means enters the cladding and is absorbed by the buffer coating and lost. If the buffer coating is removed, light may enter the cladding and be transmitted in a number of ways, or modes, through the uncoated cladding depending on the angle of the light entering the cladding. Any light in the cladding surrounded by the buffer coating will be lost due to absorption.
Band-elimination filters have been realized by causing certain wavelengths of light to be eliminated, or removed, to exit the core and enter the cladding. Once these wavelengths encounter the buffer coating, they will be removed by eliminating total internal reflection thereby allowing light in the cladding to pass into and out of the buffer coating. Physical gratings have been impressed into optic fibers to cause light of a particular wavelength in the core to enter the cladding and be removed. Microbending of the optic fiber is another approach for causing light of a particular wavelength to exit the core and enter the cladding. It is also known that an acoustic wave may be applied to an optic fiber to cause light of a particular wavelength in the core to enter the cladding and be removed.
An acoustic wave may be induced in an optic fiber by applying an alternating current (AC) electrical signal to an acoustic (e.g., piezoelectric) transducer attached to a horn which is further attached to the cladding of the optic fiber. The horn focuses the acoustic energy generated by the acoustic transducer onto the optic fiber. The horn is made of a material that matches the acoustic impedance of the optic fiber. Typically, fused silica is used as the horn. The horn is attached to the optic fiber using glue. The buffer coating is removed to allow light to be transmitted via the cladding modes for the length that the buffer coating is removed. An acoustic wave induced in an optic fiber causes the fiber to flex at a frequency equal to the frequency of the modulating electrical signal applied to the acoustic transducer. The flexural acoustic wave propagates along the optic fiber with low loss if the optic fiber remains straight.
In an article by Helge E. Engan et al., entitled "Propagation and Optical Interaction of Guided Acoustic Waves in Two-Mode Optical Fibers," published by the Institute of Electrical and Electronics Engineers, Inc. (IEEE) in The Journal Of Lightwave Technology, March 1988, Vol. 6, No. 3, pp. 428-436, it is disclosed that a flexural acoustic wave may be induced in a optic fiber using a glass rod and an acoustic transducer. FIG. 1 is an illustration of the device 1 of this article, where a silica horn 2 is attached to an optic fiber 3. Typically, an optic fiber 3 includes a core (not shown), a cladding (not shown) over the core, and a buffer coating 4 over the cladding. The optic fiber 3 does not include a buffer coating 4 over which it is desired to induce a flexural acoustic wave. The silica horn 2 is attached to, and driven by, an acoustic transducer 5. The acoustic transducer 5, which may be a piezoelectric device, is controlled electronically by a signal generator 6.
The device 1 may be operated by applying an optic signal to the core of the optic fiber 3. A flexural acoustic wave may be induced in the optic fiber 3 by the silica horn 2 under control of the acoustic transducer 5 and the signal generator 6. The frequency of the signal generator 6 determines which wavelengths of the optic signal exit the core and enter the cladding. Any part of the optic signal in the cladding and in contact with the buffer coating 4 is eliminated by causing the cladding mode light to escape.
U.S. Pat. No. 5,007,705, entitled "VARIABLE OPTICAL FIBER BRAGG FILTER ARRANGEMENT," discloses a filter that uses a permanent Bragg grating and a grating induced by a flexural acoustic wave to eliminate unwanted wavelengths in an optic signal. The present invention does not use a permanent Bragg grating or a flexural acoustic wave to eliminate unwanted wavelengths. U.S. Pat. No. 5,007,705 is hereby incorporated by reference into the specification of the present invention.
In an article by T. A. Birks et al., entitled "Four-port fiber frequency shifter with a null taper coupler," published by the IEEE in Optics Letters, Dec. 1, 1994, Vol. 19, No. 23, pp. 1964-1966 and in a correction published by the IEEE in Optics Letters, Feb. 1, 1995, Vol. 21, No. 3, page 231, it is disclosed that a flexural acoustic wave may be induced in a optic fiber using a glass horn and an acoustic transducer to eliminate unwanted wavelengths in an optic signal to realize various filters. The present invention does not use a flexural acoustic wave to eliminate unwanted wavelengths.
In an article by Seok Hyun Yun et al., entitled "All-fiber tunable filter and laser based on two-mode fiber," published by the IEEE in Optics Letters, Jan. 1, 1996, Vol. 21, No. 1, pp. 27-29, it is disclosed that a flexural acoustic wave may be induced in an optic fiber using an in-line coaxial glass horn and an acoustic transducer to eliminate unwanted wavelengths in an optic signal. The present invention does not use a flexural acoustic wave to eliminate unwanted wavelengths. FIG. 2 is an illustration of the device 20 of this article, where a silica horn 21 is attached in-line to an optic fiber 22 which does not include a buffer coating 23 for the same reason as in FIG. 1. The silica horn 21 of FIG. 2 is attached to, and driven by, an acoustic transducer 24. The acoustic transducer 24 is controlled electronically by a signal generator 25. The device 20 operates as does the device 1 of FIG. 1. Putting the silica horn 21 in line with the optic fiber 22 makes the device 20 more durable than the device 1 of FIG. 1.
In an article by Seok Hyun Yun et al., entitled "Suppression of polarization dependence in a two-mode-fiber acoustic-optic device," published by the IEEE in Optics Letters, Jun. 15, 1996, Vol. 21, No. 12, pp. 908-910, it is disclosed that a flexural acoustic wave may be induced in an optic fiber using an in-line glass horn and an acoustic transducer to eliminate unwanted wavelengths in an optic signal. The present invention does not use a flexural acoustic wave to eliminate unwanted wavelengths.
In an article by Hyo Sang Kim et al., entitled "All-fiber acousto-optic tunable notch filter with electronically controllable spectral profile," published by the IEEE in Optics Letters, Oct. 1, 1997, Vol. 22, No. 19, pp. 1476-1478, it is disclosed that a flexural acoustic wave may be induced in an optic fiber using an in-line glass horn and an acoustic transducer to eliminate unwanted wavelengths in an optic signal. The present invention does not use a flexural acoustic wave to eliminate unwanted wavelengths.
In an article by Y. W. Koh et al., entitled "Broadband polarization-insensitive all-fiber acousto-optic modulator," published by the Optical Society of America in the Optical Fiber Conference '98 Technical Digest, Feb. 22, 1998, pp. 239-240, it is disclosed that a flexural acoustic wave may be induced in an optic fiber using an in-line glass horn and an acoustic transducer to eliminate unwanted wavelengths in an optic signal. The present invention does not use a flexural acoustic wave to eliminate unwanted wavelengths.
An article by D. S. Starodubov et al., entitled "All-Fiber Bandpass Filter with Adjustable Transmission Using Cladding-Mode Coupling," published by the IEEE in IEEE Photonics Technology Letters, November, 1998, Vol. 10, No. 11, pp. 1590-1591, discloses, as illustrated in FIG. 3, a bandpass filter 30 that includes an optic fiber 31 without a buffer coating, a first fixed grating 32, a blocker of optic signals 33, a cladding mode modulator 34, and a second fixed grating 35. The first fixed grating 32 induces wanted wavelengths of an optic signal in the core of the optic fiber 31 to enter the cladding of the optic fiber 31. The blocker of optic signals 33 is within the core of the optic fiber 31 and blocks the unwanted wavelengths in the core from propagating any further. The cladding mode modulator 34 impress a signal upon the cladding of the optic fiber 31 to cause losses in, and decreased transmission through, the cladding. The second fixed grating 35 induces the wanted wavelengths in the cladding to enter the core. The device 30 has the disadvantages of having a fixed wavelength selectability, allowing only one wavelength to pass at a time, requiring the two gratings 32, 35 to be identical which is difficult to do, requiring mechanical movement of the cladding mode modulator 34, and suffering from a high insertion loss of 2 dB. The present invention does not suffer from these disadvantages and does not use two fixed gratings 32, 35 or a mechanical cladding mode modulator 34.